1. Field of the Invention
The present invention relates generally to a piezoelectric transformer. More specifically, the invention relates to an electrode structure of a piezoelectric transformer, in which an input electrode and an output electrode are provided on a surface of an elongated plate form piezoelectric body.
2. Description of the Related Art
A high power transformer often is employed for a back-light inverter for a liquid crystal display, for example. There has been proposed a piezoelectric transformer which has a completely different operation principle relative to a winding transformer.
FIG. 11 is a perspective view of a piezoelectric transformer, disclosed in Japanese Unexamined Patent Publication (Kokai) No. Heisei 7-193293, as a typical example of the conventional piezoelectric transformer. As shown in FIG. 11, an elongated plate form piezoelectric body 1 is generally separated into three regions of equal length, namely a driving portion 5L, a power generation portion 6 and a driving portion 5R along a longitudinal axis.
On upper and lower surfaces of the driving portion 5L, electrodes 2L and 3L are provided over substantially entire surfaces of respective regions 5L. On upper and lower surfaces of the other driving portion 5R, electrodes 2R and 3R are provided similarly. During a fabrication process, the driving portions 5L and SR are polarized along an axis in the thickness direction of the piezoelectric body 1, as shown by vertically oriented arrows utilizing the upper and lower electrodes provided respectively.
On the other hand, in the power generating portion 6, at the center portion of the longitudinal axis of this region, a thin electrode 7 is provided extending in the width direction of the piezoelectric body 1 around the circumference. The power generating portion 6 is polarized in mutually opposite directions along the longitudinal axis at a portion on the driving portion 5L side and at a portion on the driving portion 5R across the electrode 7. The polarization is provided during a fabrication process by maintaining the piezoelectric body, on which the electrodes are formed, at high temperature higher than or equal to about 150.degree. C. and by applying a direct current high electric field in the extent of 2 kV/mm to the driving portion and the power generating portion.
At this time, application of necessary electric field is performed in the following manner. Namely, two polarizations are different in polarizing directions, such as thickness direction and longitudinal direction of the piezoelectric body 1, and thus are performed separately. Upon polarization of the driving portion, the electrodes 2L and 2R on the upper surfaces of the driving portions 5L and 5R are shorted to have the same potential. Also, the electrodes 3L and 3R on the lower surface of the driving portions are mutually shorted to be the same potential.
Applying a direct current voltage between the upper electrodes 2L and 2R and the lower electrodes 3L and 3R, an electrical field along thickness axis of the piezoelectric body 1 is applied. On the other hand, upon polarization of the power generating portion 6, upper and lower electrodes 2L, 3L, 2R and 3R are connected to be the same potential. Then, by applying the direct current voltage between the electrode 7 of the power generating portion and the electrodes 2L, 3L, 2R and 3R of the driving portion, mutually opposite directions of electric fields along the longitudinal axis across the electrode 7 are provided by application of the voltage for the piezoelectric body 1.
Boosting a transforming operation in the piezoelectric transformer is performed in the following manner. At first, the electrodes 2L and 2R on the upper surfaces of two driving portions are connected to have the same potential. Similarly, the electrodes 3L and 3R are connected in similar manner. Between upper and lower electrodes, namely between the input terminals 4A and 4B, an alternating current voltage e.sub.in is input. By appropriately selecting the frequency of the alternating current input voltage, the piezoelectric body 1 causes mechanical resonation of vertical vibration along the longitudinal axis. The power generating portion 6 due to resonating vibration of the longitudinal axis generates vibration stress corresponding to vibration displacement of the longitudinal axis. Then, by vibration stress and piezoelectric effect by polarization in the longitudinal axis, a charge is generated. Then, the transformed output voltage e.sub.out can be obtained between the electrode 7 of the power generating portion and the electrodes 2L and 2R of the driving portion, and namely between the output terminal 8A and the output terminal 8B.
On the other hand, in order to function as a piezoelectric transformer foregoing polarization along the axis in the thickness and polarization along the longitudinal axis are inherent. Furthermore, respective polarizations have to be sufficiently saturated polarization. Therefore, as set forth above, upon polarization during the fabrication process, a strong electric field about 2 kv/mm under atmosphere of 150.degree. C., for example, is applied to the driving portion and the power generating portion of the piezoelectric body. As a result, the following adverse effect should appear.
At first, in the driving portion between upper and lower electrodes 2L and 3L (in case of the left side driving portion 5L, and if in case of the right side driving portion 5R the electrodes 2R and 3R, hereinafter discussion will be given for the case of the left side driving portion 5L), dielectric breakdown occurs which easily causes discharging. This tendency is further accelerated by the following fact. Namely, the electrode on the surface of the piezoelectric body is formed by a thick film method utilizing screen printing in the most case. In this case, at the edge portions of the electrodes 2L and 3L, on the side surface of the piezoelectric body sandwiched between both electrodes, there is easily caused a "run in printing". Once a run in printing is caused, a local concentration of electric field is caused during polarization of the driving portion to easily cause discharging. Normally, polarization is performed within the insulative fluid in order to prevent discharging. However, in a certain magnitude of run in printing or purity of insulative fluid, prevention of discharging can be insufficient.
Once discharging is caused between the electrodes 2L and 3L of the left side driving portion 5L, in an extreme case, the piezoelectric body 1 experiences mechanical breakage. And, even when the extreme case does not exist so that breakage does not occur, a vibration life of the piezoelectric body, which lowers mechanical strength, becomes short. On the other hand, insulation of the input side (driving portion) becomes insufficient to prevent failure when he input voltage is applied.
Next, in the power generating portion, similar to the driving portion set forth above, there is a possibility of discharging. However, as can be seen from experience, a longer distance between the electrodes (distance between the electrodes 2L and 3L of the driving portion and the electrode 7 of the power generating portion) reduces the possibility of discharging. On the other hand, breakage of the piezoelectric body often occurs at the power generating portion during driving, after completion of the transformer. Namely, when the piezoelectric transformer is in operation as a transformer, vibration stress is constantly acting on the piezoelectric body. Normally, the vibrating condition is set so that the vibration stress becomes less than or equal to half of the mechanical strength of the material of the piezoelectric body to avoid occurrence of failure. Despite this, in case of the transformer, in which polarization is successfully completed without causing discharging, the transformer may experience breakage during operation due to the vibration stress. This phenomenon is considered to represent lowering of the mechanical strength due to certain fine damage caused during polarization.
For example, as set forth above, irrespective of either the driving portion or the power generating portion, polarization is performed utilizing the electrode provided on the surfaces in respective regions. Therefore, within a single piece of the elongated plate form piezoelectric body, a portion where mechanical strain is caused due to application of the voltage and a portion where the voltage is not applied and thus this strain is not caused, are present simultaneously. At the boundary between the portions set forth above, large mechanical stress should act. Normally, because the piezoelectric body is ceramic, it can be easily imagined that a micro crack should be caused within the piezoelectric body under the condition of application of the local stress. On the other hand, when chipping due to handling during the fabrication process is present at the edge portion of the elongated plate for the piezoelectric ceramic plate, when vibration stress acts thereon, stress concentration should be caused at the tip end of the crack of chipping to thereby cause the chipping to grow into a crack. Thus, failure in the form of a breakage during operation as transformer can be caused. In such case, breakage during operation is substantially always caused in the power generating portion, as known from experience.
In order to prevent failure associated with polarization, preventing run of a conductive paste during a screen printing process used to form the surface electrode and thus chipping of the piezoelectric body should be effective. For example, Japanese Unexamined Utility Model Publication (Kokai) No. Heisei 6-52161 discloses forming a chamfer on at least the edges along the longitudinal axis among all edges of the elongated plate form piezoelectric ceramic plate. By forming a chamfer, chipping by handling of the piezoelectric body can be prevented, and a run in printing of the electrode of the driving portion can be eliminated.
According to another measure, the shape of the electrode on the surface of the piezoelectric body (plain pattern as viewed in the direction perpendicular to the electrode forming surface) is modified to preliminary form the electrode to a position slightly offset from the edge of the piezoelectric body inwardly. For example, as shown in FIG. 11, by extensively observing the electrodes 2L and 3L of the driving portion 5L, these electrodes are slightly offset from the edge of the piezoelectric body inwardly. By employing such an electrode structure, discharging due to a run in printing, at least upon polarization of the driving portion, and thus the breakage of the piezoelectric body can be prevented.
On the other hand, in the piezoelectric transformer shown in FIG. 11, the electrode 7 at the center or the power generating portion is a loop electrode extending over the entire circumference of the piezoelectric body. The structure of the loop electrode is not directly intended to prevent breakage of the element during driving of the piezoelectric transformer, but can be expected to prevent concentration of an electric field in the vicinity of the output electrode 7, excessive internal stress induced by concentration of the electric field, and generation of a micro-crack at the ceramic in the vicinity of the electrode 7 due to excessive internal stress, and to eliminate possibility that the mechanical strength of the ceramic and breakage of the piezoelectric body during operation of the piezoelectric transformer will be lowered.
This effect has been found by the present inventors and will be discussed hereinafter in detail.
As set forth above, in the conventional piezoelectric transformer, breakage of the piezoelectric body can be avoided by forming a chamfer on the edge of the piezoelectric body or by modifying the electrode structure. However, even in such improved transformer, the following problem can be encountered.
First, Japanese Unexamined Utility Model Publication (Kokai) No. Heisei 6-52161 discloses a transformer provided with a chamfer on the edge of the piezoelectric body. In the transformer, any effect cannot be expected for the micro-crack within the piezoelectric ceramic body generated upon polarization. The processing to form the chamfer should per se cause an increase in the fabrication process and thus serves as a factor that elevates cost. Also, due to difficulty of automating, manual process is inherent to further cause a cost increase.
Next, in the piezoelectric transformer disclosed in Japanese Unexamined Patent Publication No. Heisei 7-193293, it becomes necessary to employ soft electrode material or to form the electrode quite thinner in order to prevent interference from vibration of the electrode of the piezoelectric body. In consideration of industrial production, it should be typical to form Ag electrode or Ag--Pd alloy electrode by screen printing employing the conductive paste.
However, in this method, breakage of the electrode can be easily caused at the ampullar crista of the plate form piezoelectric body. This is because that the conductive paste can be deposited thinner at the ampullar crista. Namely, at the time of production, breakage of the electrode can be caused, or even when breakage of the electrode during fabrication can be avoided, breakage of the electrode of the ampullar crista can be caused due to vibration of transformer during driving of the transformer after completion of fabrication. In either case, breakage of the electrode is caused at two ampullar crista, a performance to concentrate a charge generated in the power generating portion of the transformer to take out, can be lowered. As a result, the transformer cannot satisfy necessary transformer output, nor can it make the transformer output during operation unstable to finally cause fatigue.
On the other hand, in case of a chamfer on the edge portion disclosed in Japanese Unexamined Utility Model Publication No. Heisei 6-52161, no effect for the micro crack within the piezoelectric ceramic generated during polarization, cannot be expected. Furthermore, forming chamfer is per se a factor which increases process steps to thereby increase cost. Also, due to difficulty of automating, a manual process is inherent to causing defects, which even further elevates costs.
Furthermore, Japanese Unexamined Patent Publication No. Heisei 4-206580 discloses a stack structure and thus has a different mechanism in the generation of a micro-crack to that of the single plate. Thus, the invention disclosed in this publication is directed to relax the stress of the interface of the stacked layers between the ceramic portion and the internal electrode portion. On the other hand, assuming that the piezoelectric ceramic transformer of the single plate structure has certain effect in stress relaxation, formation of the electrode over the entire surface is not possible to cause stable polarization for problem of printing run, particularly in the driving portion where less expensive screen printing is employed. Furthermore, even in the power generating portion, a problem of breakage of the transformer due to offset of polarization, localization of the stress associated with offset of polarization should be encountered.